High sensitivity displacement measuring device using linear variable differential transformer

ABSTRACT

The present invention is intended to provide a ultra-precision high sensitivity displacement measuring device which has such a high resolution as to be able to make submicron measurement.  
     According to the invention, there is provided a displacement measuring device with high resolution, comprising: an electromagnetic system( 10 ) which forms a closed loop of magnetic blocks( 17,19 ) and which houses primary coil bundles( 11 ) and secondary coil bundles( 13 ) for forming magnetic fields within said closed loop of magnetic blocks( 17,19 ); plate springs( 20 ) which include displacement input parts( 21 ) and displacement output zones( 24 ) fixed to the cores( 14 ) having the secondary coils wound around them and which act to guide so that the displacement output zones( 24 ) can output the displacement amplified in proportional to the displacement input to the displacement input parts( 21 ); and a supporting mechanism( 30 ) for supporting the displacement input parts( 21 ) of said plate springs( 20 ) so that the displacement may be input only in one axial direction. (FIG.  2 ).

FIELD OF THE INVENTION

[0001] The present invention relates to a linear variable differential transformer (shortly ‘LVDT’ in the following) widely used for measurement of displacements and particularly to a high sensitivity displacement measuring device using LVDT, which can measure submicron level with an insensitive to surrounding environment, noise and the like and with an increased resolution.

DESCRITPTION OF THE PRIOR ART

[0002] The sensitivity represents the ratio of the analog voltage output relative to the displacement input to be measured in a displacement measuring device using LVDT, wherein a high sensitivity, for example, means the large value for the output generated for a given displacement input.

[0003] For such an ordinary analog output producing device, the resolution is closely related with the noise and sensitivity.

[0004] Resolution=Noise/Sensitivity

[0005] In practical circumstances, the resolution is influenced severely by the noise, that is, the surrounding environment or various other factors. A high sensitivity is advantageous in order that the insensitiveness to external factors can be maintained and the resolution can be high enough to realize application in submicron area, with the minimum measurable displacement lowered.

[0006]FIG. 1 shows schematically a displacement measuring device using LVDT according to a conventional art.

[0007] As shown in FIG. 1, the displacement measuring device according to a conventional art has the form of cylinder, in which there is disposed a primary coil bundle 2 in cylindrical form, with the secondary coil bundles 3H and 3L positioned on the top and bottom thereof. In the center of the primary coil bundle 2 and secondary coil bundles 3H and 3L, there is disposed a magnetic core 4, to the lower end of which there is connected a rod 5, having a contact probe 6 disposed at its lower end. Under the lower secondary coil bundle 3L, ball bearings 8 are disposed around the rod 5 to facilitate the vertical movement of the rod 5, with the movement of which there expands or contracts a spring 9 disposed below the ball bearings 8. The primary coil bundle 2, upper and lower coil bundles 3H and 3L, ball bearings 8 and spring 9 are housed in a housing 7, under which housing the lower part of contact probe 6 protrudes.

[0008] Thus, as the contact probe 6 protruding from the bottom end of the housing 7 moves up or down depending on a displacement, the magnetic core 4 connected to the rod 5 accordingly moves up or down the same distance.

[0009] On the other hand, when an electric voltage is applied to the primary coil bundle 2, a magnetic field is generated within the displacement measuring device 1, causing the magnetic core 4 to move in accordance with the displacement input. Thus, the flux distribution in the respective secondary coil bundles 3H and 3L within the displacement measuring device 1 are changed and so the values for the difference in voltage in the form of differential voltages which are induced in the respective secondary coil bundles 3H and 3L due to the change in the magnetic fields are also changed, wherein the values for such differential voltages are proportional to the displacement inputs.

[0010] For such displacement measuring devices based on the conventional LVDT, an improvement in the sensitivity can be expected when the ratio of the windings for the secondary coil bundles to the windings for the primary coil bundle are large. Therefore, the number of the windings for the secondary coil bundles needs to be increased to increase the measurement sensitivity, with the result that the size of an overall displacement measuring device must be increased.

[0011] Furthermore, as the coil windings continuously increase, a certain limitation for coil windings is met due to capacity saturations, generation of non-linear elements or the like. Conclusively, there was a disadvantage that a high sensitivity is hardly attained.

[0012] In addition, the type of measuring device with the components of a guide such as ball bearings, springs and the like has the drawback that submicron resolution can not be attained because of the intrinsic non-linearity.

SUMMARY OF THE INVENTION

[0013] The present invention was created to resolve the above-described problems with the conventional art and the object of the invention is to provide a ultra-precision high sensitivity displacement measuring device which is improved in its construction to facilitate a ultra-precision measurement and which has such a high resolution as to be able to make submicron measurement due to increased sensitivity by using a guiding mechanism to guide and amplify an input displacement.

[0014] To that end, there is provided according to the invention a displacement measuring device with high resolution, comprising: an electromagnetic system which forms a closed loop of magnetic blocks and which houses primary coils and secondary coils for forming magnetic field within said closed loop; guiding mechanism which include displacement input parts and displacement output zones and which act to guide so that the displacement output zones can produce a displacement output amplified in proportional to the displacement input to the displacement input parts; and a supporting mechanism for supporting the displacement input parts of said guiding mechanism so that the displacement may be input only in one axial direction.

[0015] According to another aspect of the invention, there is provided a displacement measuring device, wherein said electromagnetic system includes a plurality of loop magnetic blocks in E-form, beam magnetic blocks connecting the upper and lower free ends of said loop magnetic blocks to form a closed loop, a plurality of primary coils wound around inward projections of said loop magnetic blocks, a plurality of magnetic cores which extend parallel to said beam magnetic blocks between opposite primary coils and which are positioned at a predetermined distance of less than millimeters from the end of inward projection of a loop magnetic blocks, the inward projection being wound by primary coil, and a plurality of secondary coils winding around said magnetic cores, wherein the magnetic cores are fixed, at their ends, to the displacement output zones of said guiding mechanism.

[0016] According to still other aspect of the invention, there is provided a displacement measuring device, wherein said cores and said beam magnetic blocks are of the same material.

[0017] According to still other aspect of the invention, there is provided a displacement measuring device, wherein said guiding mechanism, as plate springs with a thickness of some hundred micrometers, are so constructed that, below the lower edges of said plate springs there protrude the displacement input parts, above the displacement input parts there are positioned fixing zones, attached to the beam magnetic blocks, and on both sides of the fixing zones there are connected rotatable or tiltable zones, and that said fixing zones and said rotatable zones are provided in a symmetric manner in the upper edges as well beside the lower edges of the plate springs and displacement output zones connect the rotatable zones on the upper and lower edges of the plate springs; and wherein between a fixing zone and rotatable zones, between a displacement input part and rotatable zones and between rotatable zones and a displacement output zone, a connecting section or sections are connected, so that when a displacement is input in the displacement input parts, rotatable zones rotate around the connecting sections, as fulcrums, connecting the fixing zones and the rotatable zones to cause the displacement output as determined by a mathematical equation at the displacement output zones.

[0018] According to an aspect of the invention, there is provided a displacement measuring device, wherein said plate springs are made of beryllium-copper and are attached to both sides of said electromagnetic system so that the both plate springs may extend parallel to the closed loop of the electromagnetic system.

[0019] According to an aspect of the invention, there is provided a displacement measuring device, wherein said supporting mechanism comprises a fixing block to be attached to the electromagnetic system, a movable block to have a contact probe mounted and to be fixed to the displacement input parts of said guiding mechanism and supporting bars to connect said fixing block with said movable block.

[0020] According to still other aspect of the invention, there is provided a displacement measuring device, wherein said movable block is fixed, on its both faces, to the displacement input parts of the two sheets of plate springs on the both sides of said electromagnetic system, and said contact probe mounted on the movable block is oriented in the direction opposite to the beam magnetic blocks.

[0021] And, according to still other aspect of the invention, there is provided a displacement measuring device, wherein said supporting bars have the form of flat bars and connect the upper and lower faces of both the fixing block fixed to the bottom surface of the electromagnetic system and the movable block.

[0022] Thus, the high sensitivity displacement measuring device according to the present invention has the advantage that measurements in the submicron area can be easily made through the improvement in, for example, the coil arrangement, the closed loop structure and the decreased gaps at the opposite ends of magnetic cores, and particularly through the improvement in the output sensitivity based on the increased displacement amplification of the magnetic cores by the help of the displacement amplification mechanism of plate springs, and that the control of the contact force for the probe is possible through the adjustment of the elastic modulus for plate springs and the distances between connecting sections on the leaf springs.

[0023] Further, the present invention has another advantage that the movable block moves only in one direction, with supporting bars bearing the block, and the structure is less sensitive to the variation in external environment, i.e. noise or changing temperature thanks to the same material for both the beam magnetic blocks and the magnetic cores.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 shows the schematic view of a displacement measuring device by using a linear variable differential transformer according to a conventional art,

[0025]FIG. 2 shows the perspective view of a high sensitivity displacement measuring device according to one embodiment of the invention,

[0026]FIG. 3 shows an assembly drawing for a high sensitivity displacement measuring device shown in FIG. 2,

[0027]FIG. 4 shows a front view of an electromagnetic system of a high sensitivity displacement measuring device shown in FIG. 2,

[0028]FIG. 5 shows a front view of a guiding mechanism of a high sensitivity displacement measuring device shown in FIG. 2, and

[0029]FIG. 6 shows a perspective view for a supporting mechanism of a high sensitivity displacement measuring device shown in FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] A preferred embodiment of the invention will be described in detail below in conjunction with the accompanying drawings.

[0031] As shown in FIGS. 2 and 3, a displacement measuring device according to the invention is briefly divided into three parts, that is, an electromagnetic system 10 which includes ferromagnetic blocks, coils etc. and within which magnetic flux is generated, a supporting mechanism 30 for supporting a contact probe into which a displacement is input, and plate springs 20 which are fixed to the magnetic cores of an electromagnetic system and to the supporting mechanism 30 and which cause, as guiding mechanism, the movement of magnetic cores with an amplified output displacement relative to input displacement.

[0032] Referring to FIG. 4, the electromagnetic system 10 houses two primary coil bundles 11 to the right and left side. Between the primary coil bundles 11 there are positioned two secondary coil bundles 13, the coils of which are wound on magnetic cores 14 each having a predetermined length. The magnetic cores 14 are so arranged that air gaps 18 each smaller than a few mm may be present between the respective ends of magnetic cores 14 and the respective ends of inward projections of E-formed loop magnetic blocks 17 on which projections primary coils are wound, wherein the two magnetic cores 14 extend parallel to each other.

[0033] The primary coil bundles 11 are wound on the inward projections of two loop magnetic blocks 17 in the form of E and the spaces between the upper and lower free ends of the two loop magnetic blocks 17 are filled with beam magnetic blocks 19 connecting the loop magnetic blocks. Thus, the two loop magnetic blocks 17 and the two beam magnetic blocks 19 are connected together to form a closed loop, within which two secondary coil bundles 13 and two primary coil bundles 11 are positioned, wherein the loop magnetic blocks 17 and beam magnetic blocks 19 together with primary coil bundles 11 are stationary and on the other hand, two secondary coil bundles 13 with magnetic cores 14 are movable freely. The length of each beam magnetic blocks 19 is the same as that of a magnetic core 14 of secondary coil bundle 13 and the material for both magnetic cores 14 and beam magnetic blocks 19 are the same, so that they have the same thermal expansion. Thus, the thermal expansion and contraction for the magnetic cores 14 is balanced with that for the beam magnetic blocks 19, so that the air gaps 18 between the free ends of magnetic cores 14 and the fee ends of spool projections for primary coil bundles 11 can be maintained constant independent of temperature variations. On the other hand, when a voltage is applied to the primary coil bundles 11, a magnetic field is formed within the electromagnetic system 10. In response, the magnetic cores 14 are moved along with the input of displacement generated relative to the reference position to cause the magnetic flux permeating the coils of secondary coil bundles 13 wrapped around the magnetic cores 14 to exhibit change in proportion with respective displacements. Thus, there is induced in the secondary coils an induced electric voltage, which represents differential voltage.

[0034] On the other hand, a plate spring 20 as a guiding mechanism is made of beryllium-copper(Be—Cu) with the thickness of some hundred micrometers, as shown in FIG. 5. In particular, it is formed from etching part of a beryllium-copper plate by etching technique. The plate spring 20 so etched is formed in several zones, and is characterized in that all zones are integrally connected so that a displacement at one zone may lead to displacement at all zones of the plate spring 20. As shown in FIG. 5, the plate spring 20 is divided into respective zones which are symmetrically arranged in both the upper and lower part of the plate. According to the definition of zones employed in the illustration, the zone protruding from the middle lower edge of the spring 20 is termed as ‘A’ zone 21, the zone bounded by the reverse T-formed area in the middle lower edge is termed as ‘B’ zone 22, the zones to the right and left side of the zone 22 are termed as ‘C’ zone 23 and the zone formed in the middle part of the plate spring 20 is defined as ‘D’ zone 24. The narrow zones connecting the zone 21 and the zones 23 are termed as the first connecting sections 25, the areas between the zone 22 and the zones 23 are termed as the second connecting sections 26 and the areas connecting the zones 23 and the zone 24 are designated as the third connecting sections 27. The description for the lower part of the plate spring substantially holds for the upper part of the plate spring not specifically mentioned. The assembly and function of the plate spring so arranged will be described in detail in the following.

[0035] Two plate springs 20 so formed are attached to the front and back face of the electromagnetic system 10. Specifically, as clear from FIG. 3, the upper and lower beam magnetic blocks 19 are fixedly attached to the upper and lower ‘B’ zones 22, and the end faces of the magnetic cores 14 for two secondary coil bundles 13 are adhered to the corresponding positions of the zones 24 in the middle area of plate springs 20, whereby the two magnetic cores 14 can be maintained at a predetermined constant spacing and simultaneously the air gaps 18 between the end positions of magnetic cores 14 and the end positions of the inward projections of E- formed loop magnetic blocks 17 can be maintained constant, as depicted in FIG. 4. On the other hand, the elliptical hatched portions in FIG. 3 represent adhesive positions to combine the plate springs 20 with the electromagnetic system 10.

[0036] As shown in FIG. 6, a supporting mechanism 30 includes a stationary block 34 which is fixed to the bottom face of loop magnetic blocks 17 on one end side, a movable block 32 which is positioned at the middle portion of the bottom face of the beam magnetic blocks 19 and four supporting bars 36 connecting the stationary and movable block 34 and 32. Thus, the movable block 32 connected to the stationary block 34 through supporting bars 36 is positioned on the bottom surface of beam magnetic blocks 19, wherein the front and back faces of the movable block 32 are fixedly attached to the ‘A’ zones 21 of the plate springs 20. Therefore, the movable block 32 displaces only in vertical direction by the help of supporting bars 36 and the plate springs 20 and so causes the ‘A’ zones 21 of the plate springs 20 also to make the same amount of displacement, eventually resulting in the displacement of the ‘D’ zones 24 of the plate springs 20.

[0037] Furthermore, the movable block 32 is fixed, on its bottom surface, with a rod 38, on the free end of which a contact probe 39 is provided.

[0038] In the following, the function of the plate spring is described.

[0039] As described above, the ‘A’ zones 21 are fixed to the movable block 32, the ‘B’ zones 22 are fixed to the beam magnetic blocks 19 and the ‘D’ zones 24 are fixed to the magnetic cores 14 of the secondary coil bundles 13. In this state, when the movable block 32 makes a movement in line with the displacement input, the displacement of the movable block 32 is transmitted to the zones ‘C’ via the first connecting sections 25 on each of the plate springs 20. Because the leading areas of the zones ‘C’ 23 are connected to each other via the second connecting sections 26 of a fixing zone ‘B’ 22, the displacement is transmitted to the respective ‘D’ zones 24 through the third connecting sections 27 positioned in distal areas of the zones ‘C’ 23. In other words, given the displacement input, the zones ‘C’ transmit to the zones ‘D’ 24 the input displacement amplified in proportion to the distance between the first and second connecting section 25 and 26 as well as the distance between the second and third connecting section 26 and 27, in accordance with the principle of leverage. Accordingly, the displacement input acting on the movable block 32 is amplified through the plate springs 20, so that the magnetic cores 14 of the secondary coil bundles 13 make the movement substantially equal to the amplified displacement output.

[0040] This is expressed as the mathematical equation 1 as given below: ${\Delta \quad Z^{\prime}} = {\frac{\left( {{La} + {Lb}} \right)}{Lb} \times \Delta \quad Z}$

[0041] wherein, ΔZ′ stands for the output displacement, ΔZ does for the input displacement, La the distance between the first connecting section and the third connecting section, and Lb the distance between the first connecting section and the second connecting section.

[0042] The operation of the high sensitivity displacement measuring device constructed as above is described in detail below.

[0043] When a displacement is detected at the contact probe 39, the rod 38 moves up or down the same amount as that displacement. The movable block 32 also makes the same amount of vertical movement, so that the zones ‘A’ 21 of the plate springs 20, fixed to the movable block 32, make the same movement as the displacement. The movement of the zones ‘A’ 21 is transferred to the zones ‘C’ 23, wherein the displacement so passed down is amplified to the output displacement as determined by the above equation 1 at the third connecting sections 27 to cause the zones ‘D’ 24 to make the same movement.

[0044] With the movement of the zones ‘D’ 24, the two secondary coil bundles 13 and magnetic cores 14, which are secured to these zones, also move up or down the same distance as the amplified displacement output.

[0045] On the other hand, when an electric voltage is applied to the primary coil bundles 11, within the high sensitivity displacement measuring device 100 there is formed a magnetic field, the flux distribution is changed due to the movement of two magnetic cores 14. Such a change of the flux distribution causes the change in the voltage induced in the secondary coil bundles 13 or the value for differential voltage. Thus, the value for differential voltage is output in proportional to the amplified displacement output relative to the initial displacement input, as can be determined by the forgoing equation 1.

[0046] It is to be understood that, while the invention was described only with respect to a preferred embodiment, the invention is never restricted to that embodiment and a variety of modifications and alterations would be possible to a man skilled in the art by referring to the description or drawings presented here and within the spirit of the invention and thus those modifications or alterations are to fall within the scope of the invention, which scope should be limited only by the attached claims. 

What is claimed is:
 1. A displacement measuring device with high resolution, comprising: an electromagnetic system which forms a closed loop of magnetic blocks and which houses primary coils and secondary coils for forming magnetic field within said closed loop guiding mechanism which include displacement input parts and displacement output zones and which act to guide so that the displacement output zones can produce a displacement output amplified in proportional to the displacement input to the displacement input parts; and a supporting mechanism for supporting the displacement input parts of said guiding mechanism so that the displacement may be input only in one axial direction.
 2. The device according to claim 1, wherein said electromagnetic system includes a plurality of loop magnetic blocks in E-form, beam magnetic blocks connecting the upper and lower free ends of said loop magnetic blocks to form a closed loop, a plurality of primary coils wound around inward projections of said loop magnetic blocks, a plurality of cores which extend parallel to said beam magnetic blocks between oppositely located primary coils and which are positioned at a predetermined distance of less than millimeters from the end of inward projection of a loop magnetic blocks, the inward projection being wound by primary coil, and a plurality of secondary coils winding around said cores, wherein the cores are fixed, at their ends, to the displacement output zones of said guiding mechanism.
 3. The device according to claim 1 or 2, wherein said cores and said beam magnetic blocks are of the same material.
 4. The device according to claim 1 or 2, wherein said guiding mechanism, as plate springs with a thickness of some hundred micrometers, are so constructed that, below the lower edges of said plate springs there prude the displacement input parts, above the displacement input parts there are positioned fixing zones, attached to the beam magnetic blocks, and on both sides of the fixing zones there are connected rotatable or tiltable zones, and that said fixing zones and said rotatable zones are provided in a symmetric manner in the upper edges as well beside the lower edges of the plate springs and displacement output zones connect the rotatable zones on the upper and lower edges of the plate springs; and wherein between a fixing zone and rotatable zones, between a displacement input part and rotatable zones and between rotatable zones and a displacement output zone, a connecting section or sections are connected, so that when a displacement is input in the displacement input part, rotatable zones can rotate around the connecting sections, as fulcrums, connecting the fixing zone and the rotatable zones to cause the displacement output as determined by the following equation at the displacement output zone: ${\Delta Z}^{\prime} = {\frac{\left( {{La} + {Lb}} \right)}{Lb} \times \Delta \quad Z}$

wherein ΔZ′ stands for the oputput displacement, ΔZ stands for the input displacement, La stands for the distance between the connecting section connecting a displacement input part and a rotatable zone and the connecting section connecting a rotatable zone and a displacement output zone, and Lb stands for the distance between the connecting section connecting a displacement input part and a rotating zone and the connecting section connecting a fixing zone and a rotatable zone.
 5. The system according to claim 4, wherein said plate springs are made of beryllium-copper and are attached to both sides of said electromagnetic system so that the both plate springs may extend parallel to the closed loop of the electromagnetic system.
 6. The system according to claim 1, wherein said supporting mechanism comprises a fixing block to be attached to the electromagnetic system, a movable block to have a contact probe mounted and to be fixed to the displacement input parts of said guiding mechanism and supporting bars to connect said fixing block with said movable block.
 7. The system according to claim 6, wherein said movable block is fixed, on its both faces, to the displacement input parts of the two sheets of plate springs on the both sides of said electromagnetic system, and said contact probe mounted on the movable block is oriented in the direction opposite to the beam magnetic blocks.
 8. The system according to claim 6 or 7, wherein said supporting bars have the form of flat bars and connect the upper and lower faces of both the fixing block fixed to the bottom surface of the electromagnetic system and the movable block. 